A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and a fuel, such as hydrogen or a hydrocarbon-based material, such as methanol, ethanol, natural gas, or the like.
Representative examples of fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell which uses methanol as a fuel.
A polymer electrolyte membrane fuel cell (PEMFC) has a high energy density, but requires a fuel reforming processor for reforming methane, methanol, natural gas, or the like, in order to produce a hydrogen-rich gas as the fuel gas.
By contrast, a direct oxidation, fuel cell (DOFC) has an energy density that is lower than the polymer electrolyte membrane fuel cell, but does not need a fuel reforming processor.
A fuel cell includes a stack that generates electricity. The stack includes several unit cells stacked in a multi-layer fashion. Each of the unit cells is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (also referred to as a fuel electrode or an oxidation electrode) and a cathode (also referred to as an air electrode or a reduction electrode) with a polymer electrolyte membrane between them.
In one embodiment, a fuel is supplied to an anode and is reacted with a catalyst associated with the anode, and the fuel is oxidized to produce protons and electrons. The electrons are transferred into a cathode via an out-circuit, and the protons are transferred into the cathode through a polymer electrolyte membrane. An oxidant is also supplied to the cathode, and the oxidant, protons, and electrons are reacted on a catalyst at the cathode to produce electricity along with water.
The polymer electrolyte membrane is used to transfer the protons from the anode to the cathode during the cell operation. In addition, the polymer electrolyte membrane is used to electrically insulate the anode from the cathode, and is also used to separate a reacting gas or liquid.
A polymer electrolyte membrane should have high electrochemical stability and low ohmic loss at a high current density. In addition, the polymer electrolyte should have good separation capability of a reacting gas or liquid during the cell operation, and have strong mechanical properties and dimensional stability to form a stack.